Which hormone is primarily responsible for regulating water balance in the body?

The hormone primarily responsible for regulating water balance in the body is antidiuretic hormone (ADH), also known as vasopressin. ADH is produced in the hypothalamus and released from the posterior pituitary gland. Its primary function is to control the body's retention of water by acting on the kidneys. When ADH is released, it promotes the reabsorption of water from the filtrate in the renal tubules back into the bloodstream, reducing urine output and concentrating the urine.

This regulatory mechanism is crucial in maintaining the body's fluid balance and osmolarity. When the body is dehydrated or when blood osmolality is high, ADH secretion increases, prompting the kidneys to conserve more water. Conversely, when hydration levels are sufficient and blood osmolality is low, ADH secretion decreases, leading to increased urine production and water elimination.

In contrast, insulin plays a key role in carbohydrate metabolism and glucose regulation rather than water balance, ACTH primarily stimulates adrenal gland function and cortisol release which is involved in stress responses, and thyroid hormone influences metabolism and energy levels but is not directly responsible for water retention or balance. Thus, ADH’s specific role in water balance makes it the appropriate answer.